This invention relates to a process for preparing composite materials such as synthetic resins and concrete reinforced with carbon fibers. More particularly, the present invention is concerned with a process for preparing carbon fiber-reinforced composite materials wherein textile weaves obtained by weaving yarns made of carbon fibers are used as a reinforcer.
Carbon fiber-reinforced composite materials wherein textile weaves obtained by weaving yarns made of carbon fibers are used as a reinforcer for synthetic resins or conrete are well known.
Monofilaments made of carbon fibers used for such composite materials are as fine as 7.mu. to 10.mu.. In weaving carbon fibers, about 1000 to 6000 monofilaments are bonded to one another with a sizing agent to form a yarn. The yarns thus obtained are woven into a textile weave. Since the monofilaments constituting the above-mentioned yarn are substantially untwisted, bonding the monofilaments to one another with a sizing agent is indispensable to prevent breaking of them during weaving for attaining smooth weaving. In general, such yarns have a flat crosssection, and the dimension of the crosssection is, e.g., of the order of 2 mm in width and 0.15 mm in thickness.
Conventionally, the textile weave obtained by weaving yarns comprising aggregations of monofilaments bonded to one another with a sizing agent was impregnated as such with a synthetic resin or the like and molded into composite materials such as prepreg.
In general, when an external force acts on a composite material containing a carbon fiber, it acts on the carbon fiber as a tensile force, and the strength of the composite material is greatly influenced by the effective tensile strength of the carbon fiber. That is, the larger the content of the carbon fiber in the composite material which substantially bears the stress, the greater the extent of improvement in strength of the composite material. Further, the more parallel the stress acts on the filaments of the carbon fiber, the greater the extent of improvement in substantially strength of the composite material.
Therefore, in a composite material reinforced with a textile weave made of a carbon fiber, the more densely the textile weave is arranged in the composite material and the more parallel the external force acts on the yarns constituting the textile weave, the greater the extent of improvement in strength of the composite material.
However, a factor which spoils the strength inevitably exists in composite materials wherein textile weaves are used as a reinforcer.
Specifically, since warps and wefts constituting a textile weave interlace zigzag as shown in FIG. 3 (b), an angle of .theta. is made between the face of the textile weave 1 and the direction of the yarn 2 located between the crossing points P and Q, which leads to lowering in effective strength of the yarn 2. FIG. 6 is a graph showing how the strength of a synthetic resin plate wherein carbon fiber filaments doubled parallel are used as a reinforcer varies depending on the angle of .theta. (see FIG. 5) between the direction of application of an external force F and the direction T of the carbon fiber, and shows that an increase in .theta. brings about a rapid decrease in strength.
Further, in FIG. 2 (a) showing a plain of the textile weave 1, empty regions 4 in which yarns 2, 3 are absent are inevitably formed between the warps 2 and the wefts 3, and the presence of the empty regions 4 leads to not only lowering in density of the carbon fiber contained in the composite material but also molding with the empty regions 4 remaining unfilled with a synthetic resin or conrete, which results in molding of a composite material having an incomplete texture in which a number of bubble-like voids are present.
The widening of the intervals between the yarns 3 for the purpose of decreasing the value of .theta. in FIG. 2 (b) to improve the strength results in widening of the empty regions 4 in FIG. 2 (a). After all, in a carbon fiber-reinforced composite material wherein a textile weave made of a carbon fiber is used as a reinforcer, the carbon fiber cannot bear the stress in an ideal form, which leads to an inevitable disadvantage of lowering in strength attributable to it.